Communication control method, base station, and user terminal

ABSTRACT

A communication control method is employed in a mobile communication system including a general cell supporting PTM transmission of MBMS data and a specific cell not supporting PTM transmission of the MBMS data. The method comprises: a step A of broadcasting general cell load information from the general cell; a step B of receiving the general cell load information broadcasted in the step A, by a user terminal which receives the MBMS data from the general cell in an idle mode with the general cell selected as a cell on which to camp; and a step C of controlling cell reselection from the general cell to the specific cell by the user terminal, based on the general cell load information received in the step B.

TECHNICAL FIELD

The present invention relates to a communication control method, a basestation and a user terminal, employed in a mobile communication systemincluding a general cell supporting PTM transmission of MBMS data and aspecific cell not supporting PTM transmission of MBMS data.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project) which is a standardizationproject of mobile communication systems, standardization of eMBMS(evolved Multimedia Broadcast Multicast Service) is in progress (seeNon-patent Document 1, for example).

In such a mobile communication system, a general cell (such as amacrocell) supports MBMS and is capable of transmitting MBMS data bymulticast (Point-To-Multipoint: PTM).

In contrast, a specific cell (such as a CSG cell) not supporting MBMS isnot capable of transmitting MBMS data by PTM. Hence, studies are beingmade to transmit MBMS data by unicast (Point-To-Point: PTP) to a userterminal which establishes a connection with a specific cell.

Note that the term “cell” is used not only to indicate the smallest unitof a radio communication area, but also to indicate a function ofperforming radio communication with a user terminal.

PRIOR ART DOCUMENT Non-Patent Document

NON-PATENT DOCUMENT 1: 3GPP Contribution RP-111374, September 2011

SUMMARY OF THE INVENTION

Assumed is a situation wherein a communication environment in which ageneral cell includes a specific cell operated at a frequency differentfrom that of the general cell, a user terminal located at a point wherethe general cell overlaps the specific cell receives MBMS datatransmitted from the general cell by PTM in an idle mode.

Under such a specific situation, it is difficult for the user terminalto determine whether or not to reselect the cell on which to camp fromthe general cell to the specific cell.

Against this background, the present invention aims to provide acommunication control method, a base station, and a user terminalcapable of performing appropriate cell reselection.

A communication control method according to the present invention isemployed in a mobile communication system including a general cell(e.g., macrocell) supporting PTM transmission of MBMS data and aspecific cell (e.g., CSG cell) not supporting PTM transmission of theMBMS data. The method includes: a step A of broadcasting general cellload information from the general cell; a step B of receiving thegeneral cell load information broadcasted in the step A, by a userterminal which receives the MBMS data from the general cell in an idlemode with the general cell selected as a cell on which to camp; and astep C of controlling cell reselection from the general cell to thespecific cell by the user terminal, based on the general cell loadinformation received in the step B.

In the step C, the user terminal may perform cell reselection from thegeneral cell to the specific cell when uplink data to be sent isgenerated under a condition where the general cell load information ishigher than a predetermined threshold. The communication control methodmay further include a step D of establishing a connection with thespecific cell by the user terminal, after performing cell reselectionfrom the general cell to the specific cell in the step C.

The communication control method may further include a step E ofreceiving the MBMS data from the specific cell by PTP by the userterminal, after establishing the connection with the specific cell inthe step D.

In the step A, when transmitting the MBMS data from the general cell byPTM, the general cell may broadcast the general cell load information.

In the step A, when a load level of the general cell exceeds apredetermined level, the general cell may broadcast the general cellload information.

In the step C, when the general cell load information is not broadcastedfrom the general cell, the user terminal may select the general cell asa cell on which to camp without performing the cell reselection.

The communication control method may further include a step F ofacquiring specific cell load information via a network interface by ageneral base station managing the general cell. In the step A, thegeneral cell may broadcast the specific cell load information acquiredin the step F together with the general cell load information. In thestep B, the user terminal may receive the general cell load informationand the specific cell load information broadcasted in the step A. In thestep C, the user terminal may control cell reselection from the generalcell to the specific cell based on the general cell load information andthe specific cell load information received in the step B.

The communication control method may further include a step H ofbroadcasting specific cell load information from a specific base stationmanaging the specific cell and a step I of receiving the specific cellload information sent in the step H by the user terminal. In the step C,the user terminal may control cell reselection from the general cell tothe specific cell based on the general cell load information received inthe step B and the specific cell load information received in the stepI.

In the step C, when both of the specific cell load information and thegeneral cell load information are lower than their respectivepredetermined thresholds, the user terminal may select the general cellas a cell on which to camp without performing the cell reselection.

In the step C, the user terminal may perform cell reselection from thegeneral cell to the specific cell when the user terminal desires toreceive the MBMS data under a condition where both of the specific cellload information and the general cell load information are higher thantheir respective predetermined thresholds. The communication controlmethod may further include a step J of establishing a connection withthe specific cell by the user terminal, after the cell on which to campis switched from the general cell to the specific cell in the step C,and a step K of receiving the MBMS data from the specific cell by PTP bythe user terminal, after the connection with the specific cell isestablished in the step J.

The specific cell may be a CSG cell operated at a frequency differentfrom a frequency of the general cell.

A base station (e.g., eNB) according to the present invention isemployed in a mobile communication system including a general cellsupporting PTM transmission of MBMS data and a specific cell notsupporting PTM transmission of the MBMS data. The base station managesthe general cell and includes a transmitter (e.g., radio transceiver 110and controller 140) that broadcasts general cell load information.

A base station (e.g., HeNB) according to the present invention isemployed in a mobile communication system including a general cellsupporting PTM transmission of MBMS data and a specific cell notsupporting PTM transmission of the MBMS data. The base station managesthe specific cell and includes a transmitter (e.g., radio transceiver110 and controller 140) that broadcasts specific cell load information.

A user terminal according to the present invention is employed in amobile communication system including a general cell supporting PTMtransmission of MBMS data and a specific cell not supporting PTMtransmission of the MBMS data. The user terminal includes: a receiver(e.g., radio transceiver 210 and controller 240) that receives generalcell load information sent from the general cell when receiving the MBMSdata from the general cell in an idle mode with the general cellselected as a cell on which to camp; and a controller (e.g., controller240) that controls cell reselection from the general cell to thespecific cell based on the general cell load information received by thereceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration of an LTE system according to first andsecond embodiments.

FIG. 2 shows a protocol stack of a radio interface of the LTE systemaccording to the first and second embodiments.

FIG. 3 shows a configuration of a radio frame used in the LTE systemaccording to the first and second embodiments.

FIG. 4 shows a logical composition of an eMBMS base according to thefirst and second embodiments.

FIG. 5 shows mapping among logical channels, transport channels, andphysical channels according to the first and second embodiments.

FIG. 6 is a block diagram of a HeNB according to the first and secondembodiments.

FIG. 7 is a block diagram of an eNB according to the first and secondembodiments.

FIG. 8 is a block diagram of a UE according to the first and secondembodiments.

FIG. 9 shows an operation environment of the eNB and the UE according tothe first and second embodiments.

FIG. 10 shows cell reselection control according to the first and secondembodiments.

FIG. 11 is an operational sequence chart according to the firstembodiment.

FIG. 12 is an operational sequence chart according to the secondembodiment.

DESCRIPTION OF THE EMBODIMENTS Summary of the Embodiments

A communication control method is employed in a mobile communicationsystem including a general cell (e.g., macrocell) supporting PTMtransmission of MBMS data and a specific cell (e.g., CSG cell) notsupporting PTM transmission of the MBMS data. The method includes: astep A of broadcasting general cell load information from the generalcell ; a step B of receiving the general cell load informationbroadcasted in the step A, by a user terminal which receives the MBMSdata from the general cell in an idle mode with the general cellselected as a cell on which to camp; and a step C of controlling cellreselection from the general cell to the specific cell by the userterminal, based on the general cell load information received in thestep B.

Thus, the user terminal receiving MBMS data from the general cell in theidle mode with the general cell selected as the cell on which to campcontrols cell reselection from the general cell to the specific cell,based on the general cell load information.

For example, when the general cell is in a high load state, the userterminal assigns a higher cell selection priority for the specific cellto perform cell reselection from the general cell to the specific cell.Hence, connection establishment with the general cell in the high loadstate can be avoided, and degradation in the service quality received bythe user terminal can be suppressed.

The communication control method may further include a step F ofacquiring specific cell load information via a network interface by ageneral base station managing the general cell. In the step A, thegeneral cell may broadcast the specific cell load information acquiredin the step F together with the general cell load information. In thestep B, the user terminal may receive the general cell load informationand the specific cell load information broadcasted in the step A. In thestep C, the user terminal may control cell reselection from the generalcell to the specific cell based on the general cell load information andthe specific cell load information received in the step B.

Alternatively, the communication control method may further include astep H of broadcasting specific cell load information from a specificbase station managing the specific cell and a step I of receiving thespecific cell load information sent in the step H by the user terminal.In the step C, the user terminal may control cell reselection from thegeneral cell to the specific cell based on the general cell loadinformation received in the step B and the specific cell loadinformation received in the step I.

Thus, the user terminal receiving MBMS data from the general cell in theidle mode with the general cell selected as the cell on which to campcontrols cell reselection from the general cell to the specific cell,based not only on the general cell load information but also on thespecific cell load information. With this, the user terminal can controlcell reselection from the general cell to the specific cell, by takinginto consideration the load states of both of the general cell and thespecific cell.

For example, when one of the general cell and the specific cell is inthe high load state and the other is in the low load state, the userterminal assigns a lower cell selection priority for the one cell and/orassigns a higher cell selection priority for the other cell. Hence,connection establishment with the cell in the high load state can beavoided, and degradation in the service quality received by the userterminal can be suppressed.

In addition, in step C, the user terminal preferably controls cellreselection from the general cell to the specific cell by also takinginto account whether or not uplink data to be sent is generated (i.e.,whether or not unicast data needs to be exchanged).

Further, in step C, the user terminal preferably controls cellreselection from the general cell to the specific cell by also takinginto account whether or not continuous reception of MBMS data is desired(i.e., whether or not the user is still interested in MBMS).

First Embodiment

In the first embodiment, a description is given by taking as an examplea mobile communication system configured in accordance with the 3GPPstandard of release 10 or later (i.e., LTE Advanced).

Hereinafter, (1) outline of LTE system, (2) outline of MBMS, (3)configurations of HeNB, eNB, and UE, and (4) operations of HeNB, eNB,and UE are described in this order.

(1) Outline of LTE System

FIG. 1 shows a configuration of an LTE system. As shown in FIG. 1, theLTE system is formed of an E-UTRAN (Evolved-UMTS Terrestrial RadioAccess Network), a UE (User Equipment), and an EPC (Evolved PacketCore).

The E-UTRAN includes an eNB (evolved Node-B), a HeNB (Home evolvedNode-B), and a HeNB GW (Home evolved Node-B Gateway).

The eNB manages a macrocell and performs radio communication with the UEwhich establishes a connection with the macrocell.

The macrocell supports MBMS. To be specific, the macrocell is capable oftransmitting MBMS data by PTM (multicast). In the first embodiment, themacrocell corresponds to a general cell, and the eNB corresponds to ageneral base station.

The HeNB manages a CSG (Closed Subscriber Group) cell to which only anauthorized UE is allowed access, the CSG cell is a cell of a smallercoverage than the macrocell. The HeNB performs radio communication withthe UE which establishes a connection with the CSG cell.

The CSG cell does not support MBMS. To be specific, the CSG cell is notcapable of transmitting MBMS data by PTM. Note, however, that the CSGcell is capable of transmitting MBMS data by PTP (unicast). In the firstembodiment, the CSG cell corresponds to a specific cell, and the HeNBcorresponds to a specific base station.

In addition, the eNB and the HeNB have a radio resource management (RRM)function, a user data routing function, and a measurement controlfunction for mobility control and scheduling, for example.

Multiple HeNBs are connected to the HeNB GW, and the HeNB GW manages themultiple HeNBs.

The EPC includes MMES (Mobile Management Entities) and S-GWs(Serving-Gateways). The MME is a network entity for performing variousmobility control for the UE, and corresponds to a control station. TheS-GW is a network entity for controlling transfer of user data, andcorresponds to an exchange.

The eNBs (the HeNBs) are mutually connected over an X2 interface. TheeNB (the HeNB) is connected with the MME and the S-GW (HeNB GW) over anS1 interface. The X2 interface and the S1 interface form networkinterfaces.

The UE is a mobile radio communication device, and performs radiocommunication with a cell (serving cell) with which it establishes aconnection. In the first embodiment, the UE corresponds to a userterminal.

The UE in an idle mode (RRC idle mode) corresponding to a standby modeselects a cell on which to camp, and camps on the selected cell.Processing of changing the cell on which to camp in the RRC idle mode isreferred to as cell reselection.

The UE in a connected mode (RRC connected mode) corresponding to acommunicating state performs radio communication with the serving cell.Processing of changing the serving cell in the RRC connected mode isreferred to as handover.

The UE supports MBMS. Specifically, the UE in the RRC idle mode or inthe RRC connected mode can receive MBMS data transmitted from the eNB byPTM.

FIG. 2 shows a protocol stack of a radio interface of the LTE system.

As shown in FIG. 2, radio interface protocols are classified into layer1 to layer 3 of the OSI reference model, where layer 1 is the physical(PHY) layer. Layer 2 includes the MAC (Media Access Control) layer, theRLC (Radio Link Control) layer, and the PDCP (Packet Data ConvergenceProtocol) layer. Layer 3 includes the RRC (Radio Resource Control)layer.

The physical layer performs data coding/decoding,modulation/demodulation, antenna mapping/demapping, and resourcemapping/demapping. The physical layer provides a transmission service tothe upper layers through a physical channel. Data is transmitted throughphysical channels between the physical layer of the UE and the physicallayer of the eNB. The physical layer is connected to the MAC layer viatransport channels.

The MAC layer performs priority control of data, retransmissionprocessing by hybrid ARQ (HARQ), and the like. Data is transmittedthrough transport channels between the MAC layer of the UE and the MAClayer of the eNB. The MAC layer of the eNB includes a MAC schedulerconfigured to determine a transport format and resource blocks to beused in the uplink and the downlink. A transport format includes atransport block size, a modulation and coding scheme (MCS), and antennamapping.

The RLC layer uses the functions of the MAC layer and the physical layerto transmit data to the RLC layer on the reception side. Data istransmitted through logical channels between the RLC layer of the UE andthe RLC layer of the eNB.

The PDCP layer performs header compression/decompression andencryption/decoding.

The RRC layer is defined only in a control plane. Data is transmittedthrough radio bearers between the RRC layer of the UE and the RRC layerof the eNB. The RRC layer controls logical channels, transport channels,and physical channels according to establishment, reestablishment andrelease of the radio bearers. When an RRC connection is establishedbetween the RRC of the UE and the RRC of the eNB, the UE is in an RRCconnected mode, and if not, the UE is in an RRC idle mode.

A NAS (Non-Access Stratum) layer positioned higher than the RRC layerperforms session management, mobility management and the like.

FIG. 3 shows a configuration of a radio frame used in the LTE system.The LTE system employs OFDMA (Orthogonal Frequency Division MultiplexingAccess) in the downlink, and SC-FDMA (Single Carrier Frequency DivisionMultiple Access) in the uplink.

As shown in FIG. 3, a radio frame is formed of ten subframes aligned inthe time direction, and each subframe is formed of two slots arrangedside by side in the time direction. Each subframe is 1 ms in length, andeach slot is 0.5 ms in length. Each subframe includes multiple resourceblocks (RB) in the frequency direction, and includes multiple symbols inthe time direction. A guard interval called a cyclic prefix (CP) isprovided at the beginning of each symbol.

In the downlink, an interval of the couple of symbols at the beginningof each subframe is mainly a control region used as physical downlinkcontrol channels (PDCCHs). The remaining interval of the subframe is adata region mainly used as physical downlink shared channels (PDSCHs).

In the uplink, the two end portions of each subframe in the frequencydirection are control regions mainly used as physical uplink controlchannels (PUCCHs). A center portion of the subframe in the frequencydirection is a data region mainly used as physical uplink sharedchannels (PUSCHs).

(2) Outline of MBMS

MBMS is a bearer service for achieving broadcast transmission, and is ascheme in which MBMS data is simultaneously transmitted by a commonbearer to multiple UEs desiring to receive the MBMS data.

In the LTE system, multiple eNBs form an MBSFN (MBMS Single FrequencyNetwork), and MBMS data can be transmitted by an MBSFN transmissionscheme. The eNBs included in the MBSFN simultaneously and synchronouslysend the same signal. Accordingly, the UE can perform RF (RadioFrequency) synthesis for signals sent from the eNBs.

FIG. 4 shows a logical composition of an eMBMS infrastructure. As shownin FIG. 4, other than the network entities shown in FIG. 1, the LTEsystem includes a BMSC (Broadcast Multicast Service Center), an MBMS GW(MBMS Gateway), and an MCE (Multi-Cell Multicast Coordination Entity).

The BMSC holds MBMS data to be transmitted. The MBMS GW transmits theMBMS data held by the BMSC to the eNBs by IP (Internet Protocol)multicast. For each of the eNBs in the MBSFN, the MCE synchronizes theMBMS data or assigns a radio resource for the MBMS data.

FIG. 5 shows mapping among logical channels, transport channels, andphysical channels in the downlink. As shown in FIG. 5, the LTE systemdefines an MTCH (Multicast Traffic Channel) and an MCCH (MulticastControl Channel) as logical channels for MBMS. Additionally, the LTEsystem defines an MCH (Multicast Channel) as a transport channel forMBMS.

The macrocell (specifically, the eNB managing the macrocell) sends MBMSdata, and MBMS service information for controlling MBMS datatransmission, through the multicast channel (the MTCH and the MCCH) bymulticast.

On the other hand, the CSG cell (specifically, the HeNB managing the CSGcell) cannot use the MTCH or the MCCH. Note, however, that the CSG cellcan transmit MBMS data to a UE establishing an RRC connection with theCSG cell, through a channel such as a DTCH (Dedicated Traffic Channel)and a DCCH (Dedicated Control Channel) by unicast.

It is to be noted that the macrocell and the CSG cell send broadcastinformation through a broadcast channel (BCCH; Broadcast ControlChannel) by broadcast. Broadcast information is information such as aMIB (Master Information Block) and a SIB (System Information Block), forexample.

(3) Configurations of HeNB, eNB, and UE

(3.1) Configuration of HeNB

FIG. 6 is a block diagram of the HeNB. As shown in FIG. 6, the HeNBincludes an antenna 101A, a radio transceiver 110A, a networkcommunication unit 120A, a storage 130A, and a controller 140A.

The antenna 101A and the radio transceiver 110A are used fortransmission and reception of radio signals. The network communicationunit 120A performs communication over the network interface. The storage130A is a memory, for example, and stores therein information used forcontrol by the controller 140A. The controller 140A is a processor(CPU), for example, and performs processing in the aforementionedlayers, as well as performs various control to be described below.

The controller 140A also generates CSG cell load information indicatinga load level of the CSG cell (HeNB). A load level of the CSG cell (HeNB)refers to any of the following, for example.

-   -   Number of blocks used or usage rate of resource blocks (RB) in        CSG cell (HeNB)    -   Usage rate of memory and CPU in CSG cell (HeNB)    -   Amount of downlink data buffered in CSG cell (HeNB)    -   Number of UEs establishing RRC connection with CSG cell (HeNB)    -   Number of Received Random Access Preambles per unit time in CSG        cell (HeNB)    -   Packet Delay in CSG cell (HeNB)    -   Data Loss rate in CSG cell (HeNB)    -   Scheduled IP Throughput in CSG cell (HeNB)

In the first embodiment, the controller 140A controls the networkcommunication unit 120A so that the network communication unit 120Asends CSG cell load information to the macrocell (eNB) over the networkinterface.

The controller 140A may send the CSG cell load information to themacrocell (eNB) in response to a request from the macrocell (eNB). To bespecific, the controller 140A may return a single reply (transmission ofCSG cell load information) to a single request, or may keep sending theCSG cell load information periodically in response to a single request,until receiving a stoppage request.

Alternatively, even if no request is received from the macrocell (eNB),the controller 140A may periodically send the CSG cell load informationto the macrocell (eNB).

Otherwise, even if no request is received from the macrocell (eNB), thecontroller 140A may send the CSG cell load information to the macrocell(eNB) when the load exceeds a reference value. Specifically, thecontroller 14 OA may make a single transmission (transmission of CSGcell load information) when the load exceeds the reference value, orwhen the load exceeds the reference value, may keep sending the CSG cellload information periodically until the load falls below the referencevalue or receiving a stoppage request from the macrocell (eNB).

(3.2) Configuration of eNB

FIG. 7 is a block diagram of the eNB. As shown in FIG. 7, the eNBincludes an antenna 101B, a radio transceiver 110B, a networkcommunication unit 120B, a storage 130B, and a controller 140B.

The antenna 101B and the radio transceiver 110B are used fortransmission and reception of radio signals. The network communicationunit 120B performs communication over the network interface. The storage130B is a memory, for example, and stores therein information used forcontrol by the controller 140B. The controller 140B is a processor(CPU), for example, and performs processing in the aforementionedlayers, as well as performs various control to be described below.

The controller 140B also generates macrocell load information indicatinga load level of the macrocell (eNB). A load level of the macrocell (eNB)refers to any of the following, for example.

-   -   Number of blocks used or usage rate of resource blocks (RB) in        macrocell (eNB)    -   Usage rate of memory and CPU in macrocell (eNB)    -   Amount of downlink data buffered in macrocell (eNB)    -   Number of UEs establishing RRC connection with macrocell (eNB)    -   Number of Received Random Access Preambles per unit time in        macrocell (eNB)    -   Packet Delay in macrocell (eNB)    -   Data Loss rate in macrocell (eNB)    -   Scheduled IP Throughput in macrocell (eNB)

In the first embodiment, the controller 140B controls the networkcommunication unit 120B so that the network communication unit 120Breceives CSG cell load information for each of the CSG cells (HeNBs) inthe macrocell. Then, the controller 140B controls the radio transceiver110B so that the radio transceiver 110B broadcasts macrocell loadinformation with the CSG cell load information. The controller 140B maysend the macrocell load information and the CSG cell load information byincluding them in a SIB.

Note that the controller 140B may perform control such that themacrocell load information is sent during a period in which the loadlevel of the macrocell (eNB) exceeds a predetermined level, and themacrocell load information is not sent during a period in which the loadlevel does not exceed the predetermined level.

Moreover, the controller 140B may perform control such that the CSG cellload information is sent during a period in which the load level of theCSG cell (HeNB) exceeds a predetermined level, and the CSG cell loadinformation is not sent during a period in which the load level does notexceed the predetermined level.

(3.3) Configuration of UE

FIG. 8 is a block diagram of the UE. As shown in FIG. 8, the UE includesan antenna 201, a radio transceiver 210, a GPS (Global PositioningSystem) receiver 220, a storage 230, and a controller 240.

The antenna 201 and the radio transceiver 210 are used for transmissionand reception of radio signals. The GPS receiver 220 receives GPSsignals to acquire UE location information indicating location of theUE. The storage 230 stores therein information used for control by thecontroller 240. The controller 240 performs processing in theaforementioned layers, as well as performs various control to bedescribed below. The controller 240 also acquires UE locationinformation based on an output of the GPS receiver 220. Note that in acase where the UE does not have the GPS receiver 220, the UE locationinformation may be acquired on the basis of a radio signal received bythe radio transceiver 210.

The storage 230 stores therein a white list which is a list of GSG cellsto which the UE is allowed access (specifically, a list of CSG IDs). Thestorage 230 also stores therein GSG cell location information indicatinglocations of the GSG cells to which the UE is allowed access. The GSGcell location information is used with the UE location information in anautonomous search procedure, which is processing for determining whetheror not there is a GSG cell to which the UE is allowed access near theUE.

In the first embodiment, the controller 240 controls selection of a cellon which the UE camps (Cell Reselection) in the RRC idle mode.

Generally, the controller 240 assigns a rank to each cell according to aresult of comparison between the quality (Q_(meas, s)) of the currentcell and the quality (Q_(meas,n)) of a neighboring cell. The controller240 selects the cell of the highest rank as the cell on which to camp.Note that a neighboring cell is a cell in the neighborhood of thecurrent cell, as a matter of course. To be specific, the controller 240adds a hysteresis (Q_(Hyst)) to the quality (Q_(meas,s)) of the currentcell to calculate a rank (R_(s)) of the current cell. The controller 240otherwise subtracts an offset (Qoffset) from the quality (Q_(meas,s)) ofthe neighboring cell to calculate a rank (R_(n)) of the current cell.

Alternatively, based on a cell frequency priority (cell ReselectionPriority), the controller 240 selects the cell of the highest priorityas the cell on which to camp. Alternatively, based on the ranking resultand the priority (cell Reselection Priority), the controller 240 selectsthe cell of the highest priority as the cell on which to camp. It is tobe noted that a rank indicates the priority with which the cell isselected as the cell on which to camp, and thus may be regarded as atype of priority.

Note that the hysteresis (Q_(Hyst)), the offset (Qoffset), and thepriority (cell Reselection Priority) are information broadcasted fromthe macrocell (eNB).

Meanwhile, if a CSG cell included in the list of CSG cells to which theUE is allowed access (i.e., white list) is included in the neighboringcells, the highest priority is set for this CSG cell. In other words, ifa CSG cell included in the white list is included in the neighboringcells, the controller 240 selects the CSG cell as the cell on which tocamp. For example, if the frequency of the CSG cell is different fromthat of the macrocell, the highest priority may be set for the CSG cell.Note that if the frequency of the CSG cell is the same as that of themacrocell, the highest priority may be set for the CSG cell.

Hereinabove, a description has been given of general cell selection. Inthe first embodiment, the controller 240 controls cell reselection inthe following manner under specific conditions.

When receiving MBMS data transmitted from the macrocell by PTM in theRRC idle mode, and the frequency of the macrocell is different from thatof the CSG cell, the controller 240 sets a higher cell selectionpriority for the macrocell than for the CSG cell. On the other hand,when receiving MBMS data transmitted from the macrocell by PTM in theRRC idle mode, and the frequency of the macrocell is the same as that ofthe CSG cell, the controller 240 sets a higher cell selection priorityfor the CSG cell than for the macrocell.

Moreover, assume a case of receiving MBMS data transmitted from themacrocell by PTM in the RRC idle mode, the frequency of the macrocell isdifferent from that of the CSG cell, and uplink data to be sent isgenerated, i.e., unicast data needs to be exchanged. In this case, thecontroller 240 controls cell reselection based on the macrocell loadinformation and the CSG cell load information to appropriately select acell with which to establish an RRC connection. Here, it is preferablethat the controller 240 controls the cell reselection in considerationnot only of the macrocell load information and the CSG cell loadinformation, but also of whether or not continuous reception of MBMSdata is desired (i.e., whether or not the user is still interested inMBMS). Then, the UE establishes an RRC connection with the selected celland transits to the RRC connected mode. Details of the cell reselectioncontrol based on the macrocell load information and the CSG cell loadinformation will be given later.

(4) Operations of HeNB, eNB, and UE

FIG. 9 shows an operation environment of the eNB and the

UE according to the first embodiment.

As shown in FIG. 9, a CSG cell operated at a frequency different fromthat of the macrocell is provided inside the macrocell, and the UE inthe RRC idle mode is located at a point where the macrocell and the CSGcell overlaps with each other. Here, the UE is a member UE of the CSGcell, and is allowed access to the CSG cell.

In such a case, the UE normally detects that it is near the CSG cell bythe autonomous search procedure, for example, and sets the highestpriority for the CSG cell to select the CSG cell as the cell on which tocamp.

However, the UE desires to receive MBMS data, and in order to receiveMBMS data transmitted from the macrocell by PTM, sets the highestpriority for the macrocell to select the macrocell as the cell on whichto camp.

Assume a case where the UE receiving the MBMS data transmitted from themacrocell by PTM in the RRC idle mode detects that it is near the CSGcell having a frequency different from the macrocell and to which it isallowed access. In this case, the UE determines whether or not toperform cell reselection (cell reselection control) from the macrocellto the CSG cell, according to the following three criteria.

-   -   Macrocell load information and/or CSG cell load information        broadcasted from macrocell    -   Whether or not uplink data to be sent is generated (i.e.,        whether or not unicast data needs to be exchanged)    -   Whether or not continuous reception of MBMS data is desired        (i.e., whether or not the user is still interested in MBMS)

(4.1) Cell Reselection Control

FIG. 10 shows cell selection control according to the first embodiment.

As shown in FIG. 10, assume a case where the UE receiving MBMS datatransmitted from the macrocell by PTM in the RRC idle mode detects thatit is near the CSG cell having the frequency different from themacrocell and to which it is allowed access. Even in this case, if nouplink data to be sent is generated (this state is referred to below as“unicast unnecessary”), the UE does not perform cell reselection fromthe macrocell to the CSG cell, and maintains the RRC idle mode with themacrocell being selected. Thus, it is possible to avoid unnecessary cellreselection. In contrast, if uplink data to be sent is generated (thisstate is referred to below as “unicast necessary”), the UE performs cellreselection (and connection processing) based on the macrocell loadinformation and/or the CSG cell load information broadcasted from themacrocell.

To be specific, if one of the macrocell and the CSG cell is in a highload state and the other is in a low load state, the one cell isassigned a lower cell selection priority and/or the other cell isassigned a higher cell selection priority. Note that a cell in a highload state indicates a case where load information on the cell is higherthan a threshold. A cell in a low load state indicates a case where loadinformation on the cell is lower than a threshold, or where there is noload information on the cell (i.e., the load information is not sent).

For example, when the macrocell is in the high load state and the CSGcell is in the low load state, the UE sets the highest priority for theCSG cell to perform cell reselection from the macrocell to the CSG celland establish an RRC connection with the CSG cell. In this case, if theUE transitioned to the RRC connected mode desires to continue receptionof MBMS data, the UE can receive the MBMS data transmitted from the CSGcell by PTP (unicast).

On the other hand, when the CSG cell is in the high load state and themacrocell is in the low load state, the UE sets the highest priority forthe macrocell to establish an RRC connection with the macrocell withoutperforming cell reselection from the macrocell to the CSG cell. In thiscase, if the UE transitioned to the RRC connected mode desires tocontinue reception of MBMS data, the UE can receive the MBMS datatransmitted from the macrocell by PTM (multicast).

Thus, assume a case where the UE receiving MBMS data transmitted fromthe macrocell by PTM in the RRC idle mode detects that it is near theCSG cell having a frequency different from the macrocell and to which itis allowed access. Here, when unicast is necessary, the UEpreferentially selects the cell in a low load state from among themacrocell and the CSG cell, and establishes an RRC connection with theselected cell. Hence, establishment of the RRC connection with the cellin a high load state can be avoided, and thus degradation in the servicequality received by the UE can be suppressed.

Moreover, assume a case where unicast is necessary, both of themacrocell and the CSG cell are in a low load state, and continuousreception of MBMS data is desired (this state is referred to below as“MBMS necessary”). Here, the UE sets the highest priority for themacrocell to establish an RRC connection with the macrocell withoutperforming cell reselection from the macrocell to the CSG cell. In thiscase, the UE transitioned to the RRC connected mode can receive MBMSdata transmitted from the macrocell by PTM (multicast). Moreover, evenwhen the UE transitions from the RRC connected mode to the RRC idle modeafter establishing the RRC connection with the macrocell, the UE canreceive MBMS data (and MBMS service information) transmitted from themacrocell by PTM.

Meanwhile, assume a case where unicast is necessary, both of themacrocell and the CSG cell are in a low load state, and continuousreception of MBMS data is not desired (this state is referred to belowas “MBMS unnecessary”). Here, as in the case of a general method, the UEsets the highest priority for the CSG cell to perform cell reselectionfrom the macrocell to the CSG cell and establish an RRC connection withthe CSG cell.

Note that even when unicast is necessary, if both of the macrocell andthe CSG cell are in a high load state, the UE may not be able to receiveservice of sufficient quality by connecting to any of the macrocell orthe CSG cell. For this reason, when unicast is necessary, both of themacrocell and the CSG cell are in a high load state, and MBMS isnecessary, the UE establishes an RRC connection with the macrocell.Since the UE can receive MBMS data by PIM. in this way, requiredresources can be reduced.

Meanwhile, when unicast is necessary, both of the macrocell and the CSGcell are in a high load state, but MBMS is unnecessary, the UE mayeither establish an RRC connection with the macrocell or establish anRRC connection with the CSG cell after performing cell reselection fromthe macrocell to the CSG cell. Note, however, that in order to make iteasier for other UEs to utilize resources of the macrocell, it ispreferable that the UE establish the RRC connection with the CSG cellafter performing the cell reselection from the macrocell to the CSGcell.

(4.2) Operation Sequence

FIG. 11 is a sequence chart showing operations of the HeNB, the eNB, andthe UE according to the first embodiment. Here, operations in theoperation environment shown in FIG. 9 will be described. Specifically,the UE receiving MBMS data transmitted from the macrocell by PTM in theRRC idle mode detects that it is near the CSG cell having a frequencydifferent from the macrocell and to which it is allowed access.

As shown in FIG. 11, in step S11, the UE receives MBMS data transmittedfrom the macrocell by PTM in the RRC idle mode.

In step S12, the CSG cell (HeNB) sends CSG cell load information to themacrocell (eNB) over the network interface. Note that the CSG cell(HeNB) may send the CSG cell load information to the macrocell (eNB)over the S1 interface (via the MME).

In step S13, the macrocell (eNB) broadcasts CSG cell load informationand macrocell load information. The UE receives the CSG cell loadinformation and the macrocell load information.

In step S14, the UE performs the aforementioned cell reselectioncontrol. Specifically, the UE determines whether or not to perform cellreselection (cell reselection control) from the macrocell to the CSGcell, according to the following three criteria.

-   -   Macrocell load information and/or CSG cell load information        broadcasted from macrocell    -   Whether or not uplink data to be sent is generated (i.e.,        whether or not unicast data needs to be exchanged)    -   Whether or not continuous reception of MBMS data is desired        (i.e., whether or not the user is still interested in MBMS)

In step S15, when uplink data to be sent is generated, the UEestablishes an RRC connection with the cell selected as the cell onwhich to camp in the cell reselection control in step S14. Note that theprocedure for establishing the RRC connection includes a random accessprocedure.

Second Embodiment

In the first embodiment described above, the CSG cell (HeNB) sends CSGcell load information to the macrocell (eNB) over the network interface.

In a second embodiment, a CSG cell (HeNB) broadcasts CSG cell loadinformation over the radio interface. Note, however, that the CSG cell(HeNB) may perform control such that the CSG cell load information issent during a period in which the load level of the CSG cell (HeNB)exceeds a predetermined level, and the CSG cell load information is notsent during a period in which the load level does not exceed thepredetermined level.

FIG. 12 is a sequence chart showing operations of the HeNB, an eNB, anda UE according to the second embodiment. Here, operations in theoperation environment shown in FIG. 9 will be described. Specifically,the UE receiving MBMS data transmitted from a macrocell by PTM in theRRC idle mode detects that it is near the CSG cell having a frequencydifferent from the macrocell and to which it is allowed access.

As shown in FIG. 12, in step S21, the UE receives MBMS data transmittedfrom the macrocell by PTM in the RRC idle mode.

In step S22, the macrocell (eNB) broadcasts macrocell load information.The UE receives the macrocell load information

In step S23, the CSG cell (HeNB) broadcasts CSG cell load information.For example, the CSG cell (HeNB) sends the CSG cell load information byincluding it in a SIB. The UE uses a neighboring cell monitoringprocedure to receive the CSG cell load information broadcasted from theCSG cell (HeNB).

In step S24, the UE performs the aforementioned cell reselectioncontrol. Specifically, the UE determines whether or not to perform cellreselection (cell reselection control) from the macrocell to the CSGcell, according to the following three criteria.

-   -   Macrocell load information broadcasted from macrocell and/or CSG        cell load information broadcasted from CSG cell (HeNB)    -   Whether or not uplink data to be sent is generated (i.e.,        whether or not unicast data needs to be exchanged)    -   Whether or not continuous reception of MBMS data is desired        (i.e., whether or not the user is still interested in MBMS)

In step S25, when uplink data to be sent is generated, the UEestablishes an RRC connection with the cell selected as the cell onwhich to camp in the cell reselection control in step S24.

Other Embodiments

It should not be understood that the description and drawings whichconstitute part of this disclosure limit the present invention. Fromthis disclosure, various alternative embodiments, examples, andoperation techniques will be easily found by those skilled in the art.

Although the embodiments above describe operations of the macrocell(eNB) supporting MBMS, a macrocell (eNB) not supporting MBMS may beconfigured not to perform the operations according to the embodimentsdescribed above (i.e., operations related to load information).

The macrocell (eNB) supporting MBMS may be configured to perform theoperations according to the embodiments described above (i.e.,operations related to load information) only when it is providing theMBMS service (when it is transmitting MBMS data). In other words, themacrocell supporting MBMS may perform the operation during a period inwhich MBMS data is transmitted, and not perform the operation during aperiod in which the MBMS data is not transmitted.

In addition, although in the above embodiments the UE controls cellreselection based on both of macrocell load information and CSG cellload information, the UE may control cell reselection based only onmacrocell load information and not on CSG cell load information. Forexample, when uplink data to be sent is generated under a conditionwhere macrocell load information is higher than a predeterminedthreshold, (i.e., the macrocell is in a high load state), the UEestablishes an RRC connection with the CSG cell after performing cellreselection from the macrocell to the CSG cell. Further, afterestablishing the RRC connection with the CSG cell, the UE may receiveMBMS data from the CSG cell by PTP.

The embodiments above describe an example where the general cell is themacrocell, and the specific cell is the CSG cell.

However, the general cell may be a cell smaller than the macrocell, suchas a picocell. The specific cell may be a hybrid cell. A UE belonging tothe CSG regards the hybrid cell as a CSG cell, and a UE not belonging tothe CSG regards the hybrid cell as an open cell. Hence, a hybrid cellcan be considered as a type of a CSG cell.

Moreover, although the embodiments above are explained by taking the LTEsystem as an example, the present invention is also applicable to othercommunication standards such as the UMTS (Universal MobileTelecommunication System).

This application claims the benefit of priority from U.S. ProvisionalApplication No. 61/591462 (filed on Jan. 27, 2012), the entire contentsof which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As described, the present invention is useful in radio communicationsfield such as mobile communications.

1-14. (canceled)
 15. A communication control method employed in acommunication system including a first radio communication apparatussupporting PTM transmission of MBMS data and a second radiocommunication apparatus not supporting PTM transmission of the MBMSdata, comprising: a step of transmitting load information of the secondradio communication apparatus, by the second radio communicationapparatus; a step of receiving the load information from the secondradio communication apparatus, by a user terminal which receives theMBMS data from the first radio communication apparatus in an idle modewith the first radio communication apparatus selected as a communicationtarget of the user terminal; and a step of controlling a reselection ofthe communication target from the first radio communication apparatus tothe second radio communication apparatus by the user terminal, based onthe load information received from the second radio communicationapparatus.
 16. A user terminal employed in a communication systemincluding a first radio communication apparatus supporting PTMtransmission of MBMS data and a second radio communication apparatus notsupporting PTM transmission of the MBMS data, comprising: a receiverthat receives load information of the second radio communicationapparatus from the second radio communication apparatus when receivingthe MBMS data from the first radio communication apparatus in an idlemode with the first radio communication apparatus selected as acommunication target of the user terminal; and a controller thatcontrols a reselection of the communication target from the first radiocommunication apparatus to the second radio communication apparatusbased on the load information received by the receiver.
 17. A deviceprovided in a user terminal employed in a communication system includinga first radio communication apparatus supporting PTM transmission ofMBMS data and a second radio communication apparatus not supporting PTMtransmission of the MBMS data, executing: a process that receives loadinformation of the second radio communication apparatus from the secondradio communication apparatus when receiving the MBMS data from thefirst radio communication apparatus in an idle mode with the first radiocommunication apparatus selected as a communication target of the userterminal; and a process that controls a reselection of the communicationtarget from the first radio communication apparatus to the second radiocommunication apparatus based on the received load information.